A lithium pack is only as good as its worst cell, and cells in series drift apart with every cycle. This project builds a battery management system for a four-cell pack that measures each cell individually rather than the pack as a whole, bleeds charge off the cells that run ahead so the pack stays matched, estimates state of charge by counting coulombs in and out with an open-circuit-voltage correction to stop the estimate drifting, and disconnects the pack through a MOSFET switch on over-voltage, under-voltage, over-current or over-temperature.
Not just the parts. Teams budget from a component list, then run out in week six on wire, connectors and the driver board they destroyed. These are the lines that catch them.
| Line | Cost |
|---|---|
Components and materials From the bill of materials | Rs 3,067 to Rs 8,240 |
Spares and replacements 15 % — you will destroy something | Rs 460 to Rs 1,236 |
Consumables and hardware Wire, connectors, headers, screws, heatshrink | Rs 368 to Rs 989 |
PCB fabrication | Rs 350 to Rs 900 |
Report, printing and binding Two or three bound copies, plots and the plagiarism check | Rs 400 to Rs 1,200 |
| Total | Rs 4,645 to Rs 12,565 |
| Each, split 3 ways | Rs 1,548 to Rs 4,188 |
Indicative bands last checked in August 2026 — a guide, not a quotation. Prices move and vary by seller, and anything you can borrow from the lab comes straight off the total. The 4.8 week figure assumes 11 person-weeks of work shared between 3; students past 4 add coordination rather than output, so the estimate stops improving there.
The same chain in words
Every cell voltage is measured separately. Pack voltage alone hides the problem: four cells averaging 3.7 V could be three healthy cells and one at 2.9 V.
During charging, any cell that reaches the balance threshold before the others has its bleed branch switched on, so the excess charge is dissipated as heat and the remaining cells catch up.
Current through the shunt is integrated over time to give charge in and out. That is coulomb counting, and it is accurate in the short term but drifts, because every small measurement offset accumulates.
When the pack has been at rest long enough for its terminal voltage to settle, the controller corrects the accumulated estimate against the open circuit voltage curve, which resets the drift.
Any protection limit exceeded turns the pack switch off. Charge and discharge are separate paths through back-to-back MOSFETs, so the controller can block discharge while still allowing charge, which is what an over-discharged pack needs.
This is the chapter that separates an engineering project from an assembly job. Every value below carries the formula that produced it and the assumptions it rests on — check them against your own ratings before you use them, because your motor is not this motor.
Series count sets voltage, parallel count sets capacity
Four cells at 3.7 V nominal give 14.8 V, and 4.2 V per cell fully charged gives 16.8 V. Two 2500 mAh cells in parallel per position give roughly 5 Ah.
I = V / R, then P = V x I
4.2 / 33 = 127 mA and 4.2 x 0.127 = 0.53 W. Passive balancing turns that energy into heat inside the enclosure, so the resistor rating and the airflow both matter.
t = imbalance capacity / bleed current
0.1 Ah / 0.127 A = 0.79 hours. This is why balancing runs during the charge cycle rather than as a separate operation, and why passive balancing cannot fix a badly mismatched pack in one session.
V shunt = I x R, and P = I^2 x R
A larger shunt gives a cleaner signal but wastes more power and heats up, which changes its own resistance. 50 mV full scale suits a current monitor IC with an internal gain stage.
Set from the cell manufacturer's datasheet, not from folklore
Charging a lithium cell below 0 C plates metallic lithium on the anode. That damage is permanent and it is a cause of later internal shorts, which is why the temperature check gates charging specifically.
| Item | Qty | Unit cost |
|---|---|---|
Lithium cells 18650, 2500 mAh, with a documented datasheet | 8 | Rs 180 to Rs 450 |
Cell holders and nickel strip 4S2P holder, pure nickel strip, balance leads | 1 | Rs 250 to Rs 700 |
Microcontroller board ESP32 or STM32 with a multi-channel ADC | 1 | Rs 350 to Rs 900 |
Current monitor INA226 with a 5 mOhm, 2 W shunt | 1 | Rs 220 to Rs 600 |
Balance MOSFETs Logic level N-channel, one per cell, with gate drive | 4 | Rs 15 to Rs 60 |
Bleed resistors 33 Ohm, 1 W, one per cell | 4 | Rs 8 to Rs 25 |
Pack switch Back-to-back N-channel MOSFETs rated 30 A, with a driver | 1 | Rs 180 to Rs 600 |
Thermistors 10 kOhm NTC, one per cell pair plus one on the switch | 3 | Rs 15 to Rs 50 |
Display 0.96 inch OLED, or Bluetooth to a phone | 1 | Rs 180 to Rs 450 |
Fuse and holder Fast-blow pack fuse, rated below the wiring | 1 | Rs 60 to Rs 200 |
Enclosure Vented ABS box with insulated terminals | 1 | Rs 250 to Rs 700 |
Specifications are written as ratings rather than brands, so any equivalent part works. Where a line says to borrow from the lab, do — it is usually the largest number on the page, and the total drops to Rs 4,645 at the low end without them.
None of these blocks you — they are learnable inside a semester. Each one you do not have costs a couple of weeks, so count them into your plan.
Anything marked required is a blocker, not a difficulty. Arrange the access before you commit to the project, not after.
Use these when your guide says the scope is too small, or when you want something in the results chapter that nobody else in your batch will have.
The structure below is what a project report is marked against, with the mistake that costs marks in each chapter. There is no template to download and there will not be one: the writing is your work, and a site that did it for you would be helping you cheat rather than helping you pass.
A mini project report runs to about 40 to 50 pages, and the literature survey is usually brief. Guides expect one clear objective, met and measured. A final year report is normally 60 to 90 pages with a substantial literature survey, and many departments expect a paper submission alongside it.
The full report guide — front matter, conventions and page countsOne paragraph covering what the problem is, what you built, and what the result was. Written last, even though it appears first.
Loses marks: Describing the problem and the method but never stating a result. If there is no number in your abstract, it is not finished.
Why the problem matters, the context it sits in, and a clear statement of the objectives your project set out to meet.
Loses marks: Objectives written so vaguely that nobody can tell at the end whether they were met. Write objectives you can tick off in the conclusion.
What has already been done, what each approach achieved, and the specific gap your work addresses.
Loses marks: A list of paper summaries with no comparison and no gap. The survey exists to justify your design choice, so end it by saying what you chose and why.
The system as a whole: the block diagram, what each block does, and the signal or power path between them.
Loses marks: A block diagram copied from a datasheet or another report. Draw yours to match what you actually built, including the parts that are inconvenient.
Every component value and rating, with the formula, the assumptions and the arithmetic that produced it. This is the chapter that distinguishes an engineering project from an assembly job.
Loses marks: Component values with no justification. If a resistor is 10 kOhm, the report must say why it is not 1 kOhm.
The circuit as built, the PCB or wiring, the flowchart, and the parts of the code that carry the actual logic.
Loses marks: Twenty pages of pasted source listing. Include the flowchart and the few functions that matter, and put the full listing in an appendix.
What you measured, in tables and graphs, compared against what you predicted in the design chapter — including where they disagreed.
Loses marks: Photographs of the working model presented as results. A photograph is evidence that it exists, not evidence that it works. Results are measurements.
Whether each objective was met, what the measured performance was, and what the honest next step would be.
Loses marks: Future scope written as a wish list of unrelated features. It should follow from a limitation you actually hit.
Every source in a consistent style, plus datasheets, full code listings and the complete bill of materials.
Loses marks: Citing a blog or a video where a datasheet or a standard exists. Cite the primary source for anything you relied on.
The demonstration is half the assessment. These are the questions this project invites, and the answers are written the way you would give them out loud.
Because the pack voltage is an average and averages hide outliers. A four cell pack reading 14.8 V could be four healthy cells, or three cells at 3.9 V and one at 3.1 V. The weak cell is the one that will be driven into over-discharge first and over-charge first, and it is the one that determines the life of the whole pack.
Passive balancing bleeds charge off the cells that are ahead through a resistor, so the energy is wasted as heat and the pack settles at the level of the weakest cell. Active balancing moves that charge into the weaker cells using a converter, so it is far more efficient and faster, at the cost of much more circuitry. Passive is the norm for small packs because it is cheap and reliable.
Because it is an integrator with no feedback. Any offset in the current measurement, however small, is integrated continuously and accumulates without bound. It is accurate over minutes and unreliable over days, which is why it must be corrected periodically against an absolute reference such as the open circuit voltage after the pack has rested.
At low temperature the graphite anode cannot accept lithium ions quickly enough, so instead of intercalating they deposit as metallic lithium on the anode surface. That plating is largely irreversible, permanently reduces capacity, and grows dendrites that can eventually pierce the separator and cause an internal short. It is why a BMS gates charging on temperature specifically, and allows discharge over a wider range.
A MOSFET has an intrinsic body diode that conducts in one direction regardless of the gate, so a single device can block current only one way. Two devices connected source to source have their body diodes opposing, so the pair blocks in both directions and lets the controller allow charge while blocking discharge, or the reverse.
Because temperature is not uniform. Cells in the middle of a pack run hotter than those at the edges, and the switching MOSFETs are a separate heat source entirely. A single sensor in the wrong place reports a comfortable temperature while a cell in the middle is well above its limit.
Normally taken as a B.E. mini project (5th/6th sem) or B.E. final year project (7th/8th sem) project. If yours is a different course, the extensions above scope it up and the simpler half of the design scopes it down — the arithmetic does not change. Back to all projects.